Urine culture: what it is, what it tells you and when it’s tested

A urine culture looks for bacteria in your urine. The lab puts a little urine on a culture plate and checks whether bacteria grow, which ones and how many. If it finds any, the antibiogram shows which antibiotics work.

When it’s tested

Doctors order it when they suspect a urinary tract infection and want to know which bacterium is behind it. This happens mostly with unusual symptoms, fever or pain in your side, cystitis that keeps coming back or does not clear with treatment, in men and in pregnancy.

A simple cystitis in a woman who is otherwise well is often treated without a culture. In pregnancy, though, it is done even without symptoms, because there bacteria are treated.

It is not a test for people who feel well: finding bacteria without symptoms is common and usually does not need treatment. That is why Rosa does not run it. The Rosa panel includes a urinalysis, which can show signs of infection such as leucocytes and nitrites, but not which bacterium is there.

What it tells you, and what it does not

It shows whether there is a bacterium in your urine, which one and how much of it. The antibiogram then shows which antibiotics work against it.

On its own it does not say whether you have an infection. Symptoms matter: with burning and fever a bacterium is treated, without symptoms often not.

If many different bacteria grow, the sample was usually contaminated while it was collected. In that case it is repeated.

That is why it is read together with other tests:

• Urinalysis: Leucocytes and nitrites point to an infection
• White blood cells: In the blood they rise with a more serious infection
• High-sensitivity CRP: High with fever, it points to a kidney infection
• Creatinine: Shows whether your kidneys filter well

How to read the result

Negative urine culture: no bacteria grew, or so few that they do not count.

Positive urine culture: the report names the bacterium, for example Escherichia coli, and the count in CFU/mL. A count of 100,000 CFU/mL or more is the classic sign that the bacteria really come from your bladder, not from a contaminated sample. If you have symptoms, your doctor may consider an infection even with lower counts.

Antibiogram: next to each antibiotic there is a letter. S means it works, R means the bacterium resists it. Your doctor explains the other codes and chooses the antibiotic.

If your urine culture is positive or negative

• Positive urine culture: There is a bacterium in your urine. If you have symptoms, your doctor picks the antibiotic from the antibiogram. Without symptoms, it often needs no treatment.
• Negative urine culture: No bacteria that count. If your symptoms persist, your doctor looks for other causes.

How to collect urine for a urine culture

A well collected sample saves you a repeat test. It usually goes like this:

• Get a sterile pot from the pharmacy or the lab.
• Use your first urine of the morning, or at least urine that has been in your bladder for a few hours.
• Wash your hands and genitals with soap and water, no disinfectant, and rinse well.
• Let the first stream go into the toilet, then catch the midstream urine in the pot without touching the inside.
• Close the pot straight away and take it to the lab within a couple of hours. If you cannot, keep it in the fridge until then.

If you can, collect it before you start an antibiotic. Your lab may have slightly different instructions: follow them.

What can skew the result

The result depends a lot on how the urine was collected and stored.

• A contaminated sample: If urine touches your skin or the inside of the pot is touched, bacteria grow that do not come from your bladder.
• Urine left out of the fridge: At room temperature bacteria multiply, and the count looks higher than it really is.
• An antibiotic already started: It can make a culture come back negative even when there is an infection.
• Your period: Blood can contaminate the sample. If you can, wait until your period is over, or ask the lab what to do.

When to worry

Talk to your doctor if:

• You have burning, need to pass urine often, or see blood in your urine.
• You are pregnant and have urinary symptoms, or a check culture comes back positive.
• You are a man and have urinary symptoms.
• Symptoms come back after treatment, or you have several bouts of cystitis in a few months.
• You have a high fever with chills, pain in your side or back, vomiting, or you feel very unwell or confused: call 112 or go to the emergency department.

Frequently asked questions

How is a urine culture done? You collect midstream urine, ideally your first of the morning, in a sterile pot after washing with soap and water. Then you take it to the lab soon, or keep it in the fridge until then.

What does a count of 100,000 CFU/mL mean? That many bacteria of one kind grow in your urine. From 100,000 CFU/mL up, the bacteria usually really come from your bladder. Whether it is an infection to treat depends on your symptoms: your doctor decides.

Positive urine culture without symptoms: does it need treatment? Usually not. Finding bacteria without symptoms is common, and treating them does not help. The main exceptions are pregnancy and some procedures on the urinary tract: there your doctor treats them.

What is the difference between a urinalysis and a urine culture? A urinalysis reads protein, sugar, blood, leucocytes and nitrites. A urine culture grows the bacteria to find out which one is there and which antibiotic works.

Can I do a urine culture on my period? Better to wait until it is over, because blood can contaminate the sample. If the test cannot wait, ask the lab how to collect it.

Does Rosa do a urine culture? No. Rosa includes a urinalysis, which can show signs of infection, and the urine albumin test. If your doctor suspects an infection, they order a urine culture separately.

Sources

• European Association of Urology, guidelines on urological infections (asymptomatic bacteriuria, cystitis, when to culture). EAU Guidelines on Urological Infections
• Nicolle and colleagues, clinical practice guideline for the management of asymptomatic bacteriuria: 2019 update by the Infectious Diseases Society of America. Clinical Infectious Diseases, 2019. PMID 30895288
• Kouri and colleagues, the EFLM European Urinalysis Guideline 2023. Clinical Chemistry and Laboratory Medicine, 2024. PMID 38534005